The binding domain of the chicken leptin receptor [chLBD (chicken leptin-binding domain)], subcloned from the full-size chicken leptin receptor and prepared in an Escherichia coli system, was subjected to site-directed mutagenesis to identify the amino acids involved in leptin binding. A total of 22 electrophoretically pure, >90% monomer-containing mutants were expressed, refolded and purified. The effects of the mutations were tested by the ability to form complexes with ovine leptin, and the kinetic parameters of interaction were determined by surface plasmon resonance. Six mutants were used to determine whether mutations of several amino acids that differ between chLBD and mammalian LBDs will affect affinity: none showed any such effect, except the mutant A105D (Ala105→Asp), which exhibited some decrease in affinity. Surface plasmon resonance analysis identified six mutants in which binding activity was totally abolished (F73A, Y14A/F73A, V76A/F77A, L78A/L79A, V76A/F77A/L78A/L79A and A105D/D106V) and six mutants (Y14A, R41A, R41A/S42A/K43A, V103A, V135A/F136A and F136A) in which affinity for the hormone was reduced, mainly by increased dissociation rates. Gel-filtration experiments indicated the formation of a 1:1 ovine or human leptin–chLBD complex with a molecular mass of approx. 41 kDa. Gel-filtration experiments yielded 1:1 complexes with those mutants in which affinity had decreased, but not with the six mutants, which had totally lost their binding capacity. Modelling the leptin–chLBD complex indicated that the binding domain of the latter is located mainly in the L3 loop, which contributes nine amino acid residues interacting with leptin. Contact-surface analysis identified the residues having the highest contribution to the recognition site to be Phe73, Phe77 and Leu79.